Multiplex telephony system



Patented Aug. 10, 1943 UNITED STATES PATENT oFFica MULTIPLEX TELEPHONYSYSTEM I Johannes Jacobus Zaalberg van Zelst, Eindhoven,

Netherlands, assignor, by mesne assignments, to Hartford National Bankand Trust Company,

v Hartford, Conn, as trustee Application February 28, 1940, Serial No.321,342

' In Germany February 20, 1939 3 Claims.

- lvlyinvention relates to multiplex telephony systems in which aplurality of telephonic conversations are simultaneously transmittedover a common transmission medium for instance a common transmissionline.

In such systems mechanical or electrical-commutator means, for instancea cathode ray tube, is employed to successively and periodically connecteach speech channel to the common transmission medium for a very shorttime period. Thus, the conversation of each speech channel istransmitted in theform of periodically-occurring short impulses whoseamplitude is modulated in accordance with the conversation beingtransmitted. In this case the frequency of the impulses is the same foreach speech channel but the impulses of each channel are shifted withrespect to each other, v

I have found that in such systems difficulties are caused by the factthat the impulses are distorted due to the frequency-dependentattenuation of the transmission medium i. e. cable.

The object of my invention is to overcome the above diificulty and forthis purpose I provide means to make the transmission medium or thespeech channel impassable, at least for voltages below a definitethreshold value, during the time periods between successive impulses.

I prefer to make the transmission medium impassable by using asuppressor to which is supplied a voltage whose frequency is determinedby the successive impulses being transmitted and which periodicallyremoves the blocking of the suppressor. According to another embodiment,I use a device whose threshold value is so selected that the disturbingvoltagesoccurring during the time periods between successive impulses donot exceed this threshold value.

In order that the invention may be clearly un- 15 the preceding impulsesI.

traversing the cable are different for the various frequencies containedin each impulse. Furthermore, as the cable has a frequency-dependentattenuation, the impulses are strongly distorted. 5 In Figure 2 thesolid line indicates the distortion 5, indicated in dotted lines in thisfigure. Due

. to this, the magnitudes of these latter impulses are effected by themagnitude of impulse I so that each impulse 2, for instance, isenlargedto a degree which depends upon the magnitude of I This efiect causescross-talk between the various channels.

When the transmission takes place through a relatively short cable theabove described eflect is less, so that by a proper selection of thelength of the cable, the impulses shown in Fig. 1 can be distorted to adegree corresponding to that shown in Fig. 4 in which each impulse isonly slightly influenced by the preceding impulse. However, when theimpulses are to be transmitted over a long distance as is usually thecase in practice,

the distortion introduced in the first part of the cable must be removedordecreased before they can be transmitted to the next part of thecable.

To overcome this difiiculty I make the transmission medium or the speechchannel impassable, at least for voltages below a definite threeholdvalue, during each time period between two successive impulses. This,will be explained in more detail below. I

As stated, Fig. 1 shows the impulses as they are emitted at thebeginning ofthe cable and Figure 4 shows them as they reach the firstintermediate station. The distortion has now proceeded to a much smallerextent than in Fig. 2 so that, for

derstood and readily carried into effect, I shall 40 instance, the tail1 of impulse I has scarcely describe the same in connection with theprior art and with reference to the accompanying drawing in which,

Figures 1 to 9 are voltage-time graphs for use penetrated the impulse 2.

As will be described in more detail hereinafter, the first intermediatestation may comprise a suppressor which is controlled by the voltage indescribing the principle of the invention, and 5 illustrated in Fig. 3.At the moments at which Figs. 10 and 11 are schematic diagrams ofportions of systems embodying the present invention. Fig. 1 shows aseries of impulses I, 2, 3, 4 and I5 as they are transmitted andcorresponding to the the impulses I, 2, 3, 4 and 5 shown in Fig. 4 occurthe suppressor receives, for instance the impulses II, I2, I3, l4 and I5of Fig. 3. At these moments the suppressor is passable, whereas it isimfirst, second, third, fourth and fifth conversation passablebetweenthese impulses. Consequently,

respectively. In such a telephony system a very broad frequency bandmust be transmitted through the transmission medium. When making use ofa transmission line or cable as a transduring the time elapslng betweenthe impulses II, I2, I3, I4 and iii no voltage originating from theimpulses I, 2, 3, 4 and 5 shown in Fig. 4 can be transmitted. As aresult the impulses leavmission medium the transit times required for toing the first intermediate station have the shape also subject toanother type of distortion. I particularly, due to the presence of Pupincoils,

shown in Fig. 5 and the tail 1 of Fig. 4 has completely disappeared.

After the impulses have traversed the cable between the first and secondintermediate station they reach the last-mentioned station in the formof the impulses shown in Fig. 6. There they I are again subjected,before being transmitted, to

the above described treatment until they finally reach the final stationwithout any of the impulses being influenced appreciably by precedingimpulses.

However, in addition to the distortion shown in Figs. 2 and 4, theimpulses shown in Fig. 1 are More resonance phenomena may occur so thatthe impulses shown in Fig. '7 are distorted as shown in Fig. 8 when theyreach the first intermediate station. A resonance phenomenon occurs atthe beginning and at the end of each impulse, the resonance phenomenaassociated withimpulse I being denoted by reference numerals 8 and 8 inFig. 8. The resonance phenomenon 8 may at the moat only slightly alterthe average height feature of the invention is that the device providedfor the above purpose has a threshold value of such a value that thedisturbing voltages, which occur during the time elapsing between twoimpulses, do not exceed this threshold value. In Fig. 8 this thresholdvalue is indicated by a dotted line It and only the voltages above thisthreshold voltage are transmitted to the second intermediate station.

A threshold voltage is used again at this station so that the disturbingvoltages between the impulses are made inoperative. This thresholdvoltage is indicated by the dotted line I! in Fig. 9.

The last-mentioned method may also be used for eliminating thedistortion illustrated in Fig. 4.

The two above-described methods will be more fully explained withreference to the circuit arrangement shown in Figs. 10 and 11 andavailable in an intermediate station.

The circuit arrangement illustrated in Fig. 10 comprises a dischargetube 20 having two control grids 2| and 22, an anode land a cathode II.The impulses are received over a transmission line 23 and supplied tothe control grid 2| and for this purpose the conductors of thetransmission line are connected to the primary winding of a transformer24 whose secondary winding has one end connected to the control grid 2|and its other end connected through a grid bias battery 42 to the oneterminal of cathode 4|.

When the control grid 22, for instance, is almost at cathode potential,an anode current modulated by the voltages set up by the control grid 2|will flow in tube 20, so that the amplified impulses are transmitted bya transformer 25 interposed in the anode circuit of tube 20 to atransmission line 26 leading to the next intermediate station. If, incontradistinction thereto, the control grid 22 is strongly negative withrespect to the cathode, the anode current will be suppressed so thatvoltages originating from the transmission line 23 cannot be transmittedto the transmission line 26.

asses Periodical suppression of the electronic stream in-tube 20 may beeifected, for instance by supplying to the control grid 22 apulse-shaped voltage as shown in Fig. 3. As an alternative, analternating voltage may be supplied to the control grid 22 by atransformer 21, which has its secondary winding connected betweencathode 4| and grid 22 with the interposition of a resistance .22. Thisvoltage has a large amplitude and a frequency determined by thefrequency of the succeeding impulses. When the control grid 22 is madenegative relatively to the cathode by the alternating voltage, the anodecurrent is suppressed. A grid current fiows inthe other half elapsingbetween two successive impulses so that the tails or the oscillationphenomena directly behind each impulse are not transmitted.

In Fig. 11 impulses received over a transmission line 23 are supplied toan amplifying tube 30 having a cathode 45, an anode 46 and a controlgrid 3|. For this purpose the transmission .line has one conductorconnected through a grid bias battery 32 to grid 3| and its otherconductor connected directly to cathode 45. After being amplified theimpulses are transmitted through a transmission line 38. v The controlgrid 3| is negative biased by means of the battery 32, which bias hassuch a valuethat voltages below a predetermined value are not amplifiedand that the resonance phenomena, which occur at the end of eachimpulse, are not transmitted.

Although I have described my invention with reference to specificexamples I do not desire to be limited thereto because obviousmodifications will present themselves to one skilled in this art.

What I claim is:

1. In a multiplex telephony system, a common transmission line adaptedto transmit a train of elementary pulses each representing the signal inone of a pulrality of channels, and suppressor means in saidtransmission line to block the transmission line, only during theperiods between successive pulses, against the passage of disturbingvoltages which lie below a predetermined threshold value, said meanscomprising a controlled discharge tube, and a control circuit for saidtube including a source of voltage which varies in synchronism with thepulses to thereby make the tube operative in the rhythm of the pulses.

2. In a multiplex telephony system, a common. transmission line adaptedto transmit a train of elementary pulses each representing the signal inone of a plurality of channels, and suppressor means in saidtransmission line to block the transmission line, only during theperiods between successive pulses, against the passage of disturbingvoltages which lie below a predetermined threshold value, said meanscomprising a controlled discharge tube having a plurality of controlgrids, a control circuit for one of said grids and connected to thetransmission line, and a control circuit for another of said grids andincluding a source of voltage which varies in synchronism with thefrequency of the pulses to thereby make the tube operative in the rhythmof the pulses.

3. In a multiplex telephony system, a common transmission line adaptedto transmit a train of elementary pulses each representing the signal inone of a, plurality of channels, and suppressor means in saidtransmission line to block the tarnsmission line, only during periodsbetween successive pulses, against the passage of disturband connectedto the transmission line, and a 10 control circuit for another or saidgrids and including a source or voltage varying in synchronism with thefrequency or the pulses to thereby make the tube operative in the-rhythmof the pulses and a resistance element interposed between the saidlatter grid and the said voltage source.

JOHANNES JACOIBUS ZAALBERG VAN ZELST.

